The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronics industry. Studies on lithium rechargeable batteries are actively being pursued in accordance with increased needs for batteries having high energy density for use as a power source in portable electronic instruments.
Lithium rechargeable batteries include a positive electrode and a negative electrode capable of reversibly intercalating lithium ions, and an organic electrolyte or a polymer electrolyte loaded between the positive electrode and the negative electrode. The batteries produce and store electrical energy as a result of a redox reaction caused upon intercalating/deintercalating the lithium ions at the positive electrode and the negative electrode.
Lithium metal is commonly used as the negative active material for a lithium rechargeable battery. However, the use of lithium may cause problems in that the lithium forms dendrites which can cause a short circuit in such a battery, and sometimes even an explosion. Considering these problems, carbonaceous materials such as amorphous carbon or crystalline carbon have been suggested as alternatives to the use of lithium metal.
The positive active material can be considered the most important material for ensuring battery performance and safety. Commonly used materials for the positive active material are chalcogenide compounds, examples of which include complex metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2(0<x<1), or LiMnO2. Co-based positive active materials such as LiCoO2 are widely used because of their high energy density (LiCoO2 has a theoretical capacity: 274 mAh/g) and good cycle-life characteristics (capacity retention).
However, because LiCoO2 is structurally unstable, the Li is actually retained at 50% as a form of LixCoO2(x>0.5). That is, when applying a charge voltage of 4.2V relative to the Li metal, the generated capacity is only 140 mAh/g, which is about 50% of the theoretical capacity. Therefore, in order to increase the capacity to more than 50% of the theoretical capacity, the charge voltage should be increased to more than 4.2V. In this case, the atomic value of Li in LixCoO2 becomes less than 0.5, and the phase is changed from hexagonal to monoclinic. As a result, it is structurally unstable and the capacity is remarkably deceased upon repeating the cycles.
Accordingly, in order to solve the problems, studies on positive active materials have been undertaken to find an alternative material which is stable at a high charge voltage of more than 4.2V and that has a high energy density and good cycle-life characteristics. For example, LiNixCo1−xO2(0<x<1), LiNixMn1−xO2(0<x<1), and Li(NixCo1−2xMnx)O2(0<x<1) have been suggested, and LiCoO2 and LiNiO2 derivative compounds in which elements such as Ni, Co, and Mn are substituted have also been suggested. (See Solid State Ionics, 57,311 (1992), J. Power Sources, 43–44, 595(1993); Japanese Patent Laid-open Publication No. H08-213015 assigned to SONY (1996); and U.S. Pat. No. 5,993,998 assigned to Japan Storage Battery (1997)). However, no positive active material capable of substituting LiCoO2 by only changing the composition of Ni, Co, or Mn has been suggested.